How the Brain Processes Language
What part of the brain controls speech and language?
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Midline view
The Short Answer
For most people, language relies mainly on the left hemisphere. Broca's area in the frontal lobe helps produce speech, Wernicke's area in the temporal lobe helps understand it, and the parietal lobe links sounds, written words, and meaning. These areas work as a connected network, and the right hemisphere adds tone of voice and context.
Who Does What
| Brain region | Its role in language |
|---|---|
| Temporal Lobe | Hears and decodes speech sounds, and links words to their meanings. Wernicke's area is at the back of the upper temporal lobe. |
| Frontal Lobe | Plans and produces speech. Broca's area helps assemble words into sentences, and the motor cortex moves the lips, tongue, and larynx. |
| Parietal Lobe | Links sounds to spelling and meaning; important for reading, writing, and repeating what you've heard. |
| Corpus Callosum | Connects the language-dominant left hemisphere with the right, which contributes tone of voice, humour, and reading between the lines. |
The Classic Picture
The study of language in the brain began in 1861, when French physician Paul Broca examined a patient who could say only one syllable, "tan", yet understood much of what was said to him. After the patient died, Broca found damage in the left frontal lobe - the area now named after him. In 1874, German neurologist Carl Wernicke described patients with the opposite problem: fluent speech that made little sense and poor understanding, linked to damage at the back of the left temporal lobe.
Wernicke also predicted that cutting the connection between these two areas - a bundle of fibres called the arcuate fasciculus - would leave both speaking and understanding fairly intact but make it hard to repeat what you'd just heard. Patients with this "conduction aphasia" were later found. In 1965, Norman Geschwind built these findings into an influential model of language as a set of connected areas.
The Modern View
Brain imaging and studies of stroke patients have shown that the classic model is too simple. Language involves a wide network across the frontal, temporal, and parietal lobes, and the classic areas are hubs within it. When researchers scanned Broca's own preserved specimens with MRI in 2007, they found the damage extended much deeper than the patch now called Broca's area.
An influential "dual-stream" model, proposed by Gregory Hickok and David Poeppel, describes two routes, much like those in vision. A ventral stream through the temporal lobe maps speech sounds onto meaning, and draws on both hemispheres. A dorsal stream, running from the temporal lobe through the parietal lobe to the frontal lobe, maps sounds onto the movements needed to say them, and relies mainly on the left hemisphere.
Reading is a recent cultural invention - only about 5,000 years old - so the brain can't have evolved a dedicated reading area. Instead, learning to read trains part of the visual system on the underside of the left temporal and occipital lobes to recognise written words. Stanislas Dehaene and Laurent Cohen call this the "visual word form area".
Left Brain, Right Brain
Before epilepsy surgery, doctors need to know which hemisphere controls language. For decades the standard method was the Wada test, developed by Juhn Wada in the late 1940s: an anaesthetic injected into one carotid artery briefly puts one hemisphere to sleep, and if the patient stops being able to speak, that side is dominant. Today, functional MRI usually does the job.
A 2000 study by Stefan Knecht and colleagues measured language dominance in 326 healthy people. Right-hemisphere dominance was found in only 4% of strong right-handers, rising to 27% of strong left-handers. The right hemisphere isn't silent, though: it helps with the melody and emotion of speech, jokes and metaphors, and following the thread of a conversation. Split-brain patients show the division clearly - information that reaches only the right hemisphere often can't be put into words, because it can't cross the cut corpus callosum to the left.
Common Myths
"Language lives in two spots: Broca's and Wernicke's areas."
These areas are important, but language depends on a much wider network and the connections between its parts. Damage almost anywhere in that network can affect language in different ways.
"Left-handed people use the right side of the brain for language."
Most left-handers are still left-hemisphere dominant for language. Right-hemisphere language is more common in left-handers than in right-handers, but it's still the minority.
"Sign languages aren't real languages."
Sign languages have full grammars, and the brain treats them as language. A 1996 study of deaf signers found that damage to the left hemisphere caused sign language aphasias, much like spoken-language aphasias in hearing people, while damage to the right hemisphere usually didn't.
When It Goes Wrong
These conditions affect language in different ways, depending on which part of the network is involved:
- Stroke & Aphasia - Stroke occurs when blood supply to part of the brain is interrupted; when it damages language centres, it can cause aphasia - difficulty producing or understanding language.
- Split-Brain Syndrome - A set of unusual symptoms that can occur when the corpus callosum is severed or absent, causing the brain's two hemispheres to process information with reduced coordination.
- Autism Spectrum Disorder - A neurodevelopmental condition involving differences in social communication, sensory processing, and patterns of interest or behaviour, present from early childhood and varying widely in presentation and support needs.
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Sources & Further Reading
- Aphasia - National Institute on Deafness and Other Communication Disorders
- Disconnexion syndromes in animals and man - Geschwind (1965), Brain
- Paul Broca's historic cases: high resolution MR imaging of the brains of Leborgne and Lelong - Dronkers et al. (2007), Brain
- The cortical organization of speech processing - Hickok & Poeppel (2007), Nature Reviews Neuroscience
- The unique role of the visual word form area in reading - Dehaene & Cohen (2011), Trends in Cognitive Sciences
- Intracarotid injection of sodium amytal for the lateralization of cerebral speech dominance - Wada & Rasmussen (1960), Journal of Neurosurgery
- Handedness and hemispheric language dominance in healthy humans - Knecht et al. (2000), Brain
- The neurobiology of sign language and its implications for the neural basis of language - Hickok, Bellugi & Klima (1996), Nature